1. Field of the Invention
The present invention relates to an apparatus for mounting a relief plate for letterpress printing and, more particularly, to an apparatus for use in a method of letterpress printing wherein a relief plate is mounted on a carrier sheet which is then wound onto a plate cylinder for performing letterpress printing.
2. Description of the Prior Art
In preliminary operations for letterpress printing, particularly flexographic printing, a relief plate needs to be mounted on a plate cylinder with high precision, and this operation requires considerable skill and many years of experience. There have been increasing demands recently for achieving higher quality of printed matters in the art of flexographic printing, and the practice of performing multicolor printing with halftone images has expanded to evoke a need for achieving precise mounting of a relief plate onto a plate cylinder. Precision in mounting a relief plate is therefore essential for attaining printed matter of good quality. There are two methods of mounting a relief plate on a plate cylinder. One method involves directly mounting the relief plate on the plate cylinder with an adhesive, and the other involves mounting the relief plate on plastic films (hereinafter referred to as a carrier sheet) having superior dimensional stability such as a polyester film and then winding the carrier sheet onto a plate cylinder. The method of using a carrier sheet is widely utilized in the art of flexographic printing because the printing plate is easily replaced.
Two methods have heretofore been employed for mounting a relief plate on a carrier sheet: in one method, a ruling means such as a section paper is set on a table and, after a carrier sheet has been placed over the section paper, the relief plate is mounted on the carrier sheet, with marks being made on the basis of the calculation of the correct mounting positions and the estimated degree of elongation; in the other method, a mounter is used in accordance with the half-mirror method.
In the first method where the respective mounting positions are marked on the carrier sheet, an error may occur by an extent that corresponds to the thickness of the plate and considerable skill is required to achieve precise mounting. In addition, almost all of the steps of the mounting process must be carried out by a manual and hence inefficient method.
The second method requires no compensation for elongation or other factors since the respective positions for mounting a relief plate are determined with the carrier sheet being attached onto a cylinder. In this method, however, the operator performs all of his jobs looking at the image projected from the mirror and a particular mounting position will subtly vary depending upon the angle at which the image is seen. This introduces inevitable variations in reliability depending on operators. In addition, the plate cylinder that is called a dummy cylinder, used in this method is not what is set in the actual printing machine, but the dummy cylinder is employed which is equal in diameter to the latter. Therefore, if a plurality of printing machines are used, the shop is required to keep a corresponding number of dummy cylinders in stock and to replace one dummy cylinder with another which has the same diameter as that of the plate cylinder to be set in a particular machine.